This invention relates to a circuit arrangement for the selection of a frequency of signals receivable in a receiving set, suitable in particular for the selection and the tuning of a television channel in a television signal receiving set; said circuit arrangement comprises a Voltage Controlled Oscillator or VCO whose output signal has its frequency fixed by a control loop, as a function of a number N different for each of the selectable frequencies, and obtained from a storage circuit, in particular a circuit comprising a Read-Only-Memory or ROM. Circuit arrangements of this type have recently been proposed as completely electronic substitute devices for the electromechanical tuning devices (potentiometers) used hitherto in radio-television receiving sets, for the selection of the various television channels. The purpose of such electronic devices is to provide better performances, a better reliability and a tuning precision exempt from human error in connection with the manual tuning phase, as well as from thermal or other kinds of drifts.
In said electronic devices, a series of numbers corresponding to the various frequencies of the receivable signals is stored in a Read-Only-Memory (ROM) during the manufacturing of the set. To select a frequency, for instance that of a television channel, the number of the channel is set, for example by means of a ten digital key push-button panel as that of a pocket calculator, and is sent, as address, to the ROM which produces the number N corresponding to the frequency to be selected. In this way it is possible to select with considerable precision, for example, one hundred different frequencies. In fact, the precision depends only upon that of the reference sample frequency which may be taken from a fixed quartz oscillator.
However, said proposed device has two disadvantages: - the tuning obtained by selecting a fixed frequency corresponding to the theoretical frequency of the optimum tuning, is practically almost never the best one; this is due to the unavoidable calibration tolerances of the circuits of the set and to the alterations introduced into the received signal by its propagation and by the characteristics of the receiving antenna;
the user finds more inconvenience in having to remember the numbers of eight or ten channels which he usually desires to receive (numbers which have no connection between them, as for instance the numbers 2, 9, 12, 23, 31, 58 and so on) than in having to remember the correspondence between eight or ten channels and the order of as many keys or sensors, as he was accustomed to up to today.
Therefore, complete electronic devices have been proposed which are free from said disadvantages; these devices, instead of being based on a read-only-memory, are based on a memory to be filled and on a selection system which generates a ramp of voltage formed by a plurality of steps which are stored in the memory. However, this device, besides losing the advantages of precision, gives rise to inconveniences during the loading of the memory, which is a complicated operation and has to be effected in successive stages if all the desired channels are not active at the moment of loading.